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The player pilots a lone spacecraft, and must create "Sinibombs" by shooting at drifting planetoids and catching the crystals that are thereby released. Sinibombs are needed to defeat the game boss, Sinistar, an animated spacecraft with a demonic skull face. Sinistar does not exist at the start of the game, and is continuously under construction by enemy worker ships. Though time is crucial, attempting to mine too quickly will destroy a planetoid without releasing any crystals. Enemy worker ships are also gathering crystals (often stealing them from the player) which they use to construct the Sinistar. Enemy warrior ships can directly attack the player's ship. The player is given a head-start before the enemy ships have enough crystals to begin construction. Game ends when the player's ships are all destroyed.

Once the Sinistar is completely formed, a digitized voice (recorded by radio personality John Doremus[5] and played through an HC-55516 CVSD decoder[6][7]) makes various threatening pronouncements, including "Beware, I live!," "I hunger, coward!," "I am Sinistar!," "Run! Run! Run!," "Beware, coward!", "I hunger!," "Run, coward!," and various loud roaring sounds. The Sinistar has no weapon attacks, but if it contacts the player's ship while it darts about the playfield, the player's ship will be "eaten" and destroyed. A total of 13 Sinibombs are required to destroy a fully built Sinistar, although an incomplete Sinistar can be damaged to slow construction. Each short-range Sinibomb automatically targets the Sinistar when fired, but can be intercepted by a collision with an enemy ship, enemy fire, or a planetoid.

The player moves from one zone to the next each time he defeats the Sinistar. A sequence of four zones repeats continuously after the first zone. Each is named for the most numerous feature of that zone: Worker Zone, Warrior Zone, Planetoid Zone, and Void Zone (the Void Zone is especially difficult because it has very few planetoids). Beginning with the first Worker Zone, a completed but damaged Sinistar can be repaired/rebuilt by the enemy ships by gathering more crystals, extending its "lifespan" if the player is unable to kill it quickly.

Sinistar contains a bug that grants the player many lives (ships). It happens only if the player is down to one life and Sinistar is about to eat the player's ship. If a warrior ship shoots and destroys the ship at this moment, it immediately takes the player to zero lives, and Sinistar eating the player subtracts another life. Since the number of lives is stored in the game as an 8 bitunsigned integer, the subtraction from zero will cause the integer to wrap around to the largest value representable with 8 bits, which is 255 in decimal.[8]

This bug cannot be exploited if the AMOA ROMset is installed in the game. This version of the game was hurried for the 1983 AMOA Trade Show.[9] In this version of the game, the player's ship does not spin out in Sinistar's mouth when caught, but instead explodes. Therefore, a player cannot die twice.

Sinistar also contains two easter eggs, one of which displays Williams Electronics and one of which displays the programmers' names in crossword format. At the time Sinistar was made, most video game companies forbade programmers from putting their credits into the games they created, or any associated material, but programmers of some games at the time would sneak them in with easter eggs. According to R.J. Mical, Noah Falstein was worried about the trouble they might get in for sneaking an easter egg into the game and applied an unusually complex series of events to exploit the easter egg, adding that he guaranteed that no one would ever find it.

Sinistar was the first game to use stereo sound (in the sitdown version), with two independent front and back sound boards for this purpose. It was also the first to use the 49-way, custom-designed optical joystick that Williams had produced specifically for this game.[13]

In July 2000, Midway licensed Sinistar, along with other Williams Electronics games, to Macromedia Shockwave for use in an online applet to demonstrate the power of the Shockwave web content platform, entitled Shockwave Arcade Collection. The conversion was created by Digital Eclipse. It is currently freely available to be played within the Shockwave web applet.

Like most arcade games of the era, unofficial clones were made for home computers. One was Peter Johnson's Deathstar for the BBC Micro and Acorn Electron which was published by Superior Software in 1984.[14] It was originally developed as an official port to be released by Atarisoft but they decided to abandon the BBC platform while a number of games were still in development.

Some of Sinistar's iconic quotations have been included in subsequent video games. In the game Warcraft III: The Frozen Throne, the neutral hero Firelord's birth sound is him saying "Beware, I live." World of Warcraft paid tribute to the same quote: The boss enemy Reliquary of Souls shouts it when freed. "Beware - I live". It is used in the app Arcade Jumper, at the start of Zone 8, and as an achievement.

In tribute to Sinistar, the entire game was cloned as the public domain Xenostar, released for the Amiga computer in 1994. Mega Lo Mania for the Mega Drive contained a hidden Sinistar inspired mini-game accessible by entering JOOLS on the password screen.[15] In Team Fortress 2, the Heavy Weapons Guy says the phrases "I Live!" and "Run, Cowards!"

Sinistar's sound effects have been used in a number of other media projects, both for their iconic status and for their emotional effectiveness. The film We Are the Strange uses "Beware, I live", "I hunger", "Run Coward" and Sinistar's roar. The British computer game review series "BITS" used "Beware, Coward" as the ending flourish to the opening titles of every season.

A number of samples from the game have been used in song productions, such as the Buckethead song "Revenge of the Double Man" off his 1999 album Monsters and Robots which samples "Beware coward!", and "I Am Sinistar." In addition, popular internet musician Renard's alias, Furries In A Blender, has released multiple songs using many samples from the game, even going so far as to name two of his songs "Beware coward!" and "I Am Sinistar."